Lot of UDP ports opened

[Jacques Lav!gnotte <jacques@pollux.frmug.org>]

Hi there,

There are box1 and box2 on the same LAN

A daily cron does machine nmap each other.

Some times, box1 finds a lot of  opened UDP ports opened on box2.

Both machines are debian stable doing security updates.

Both rkhunter and chkrootkit are running on it and finds nothing.

Box2 nmapping herself shows a normal situation. 

A reboot turns  box2 to normal behavior.

Any  idea ?

              Thanks,           Jacques